The possibility of using phase change materials (PCMs) in nonvolatile memory cells has recently gained momentum as more is learned about these materials and their integration into integrated circuits. When incorporated in a memory cell, these materials can be toggled between higher and lower electrical resistance states by applying a pulse of electrical current (“switching current pulse”) to the memory cell. After writing to a memory cell in this way, the electrical resistance state of the given memory cell can be determined (i.e., read) by applying a low magnitude sensing voltage to the material in order to determine its electrical resistance.
Currently, binary and ternary chalcogenide alloys such as doped antimony telluride (SbTe) and germanium antimony telluride (Ge2Sb2Te5) (GST) are showing the greatest promise for use in practical PCM-based memory cells. However, the switching of a PCM-based memory cell requires that the switching current pulse produce enough heat in the PCM to cause at least some portion of the PCM to reproducibly change electrical resistance state. The required temperature may be as high 350 degrees Celsius. If the memory cell is not properly designed, the magnitude of the switching current pulse necessary to create these required temperatures can easily exceed that which can be tolerated by modern integrated circuits.
It has been recognized that the magnitude of the required switching current pulse can be reduced by forcing the current pulse to pass through an extremely narrow region of PCM. The confinement of the switching current to this narrow region results in high localized current density, and, in turn, in high localized ohmic heating. Nevertheless, it remains challenging to reliably and reproducibly form constricted regions of PCM with suitably narrow dimensions. Frequently, for example, such processing requires the formation of features that are smaller than those that can be formed using conventional photolithographic techniques. There remains, as a result, a need for methods of reliably and reproducibly forming constricted regions of PCM in PCM-based memory cells.